WASHINGTON — Billionaire Donald Trump swept all five presidential primaries held yesterday, strengthening his hold on the Republican race, while Democrat Hillary Clinton distanced herself from rival Bernie Sanders with three solid wins.

Trump defeated his rivals Ted Cruz and John Kasich in Connecticut, Delaware, Maryland, Pennsylvania and Rhode Island -- a stunning show of force by a candidate seen as a populist political saviour by millions despite being loathed by the party establishment.

On the Democratic side, US networks projected Clinton as the winner in Delaware, Maryland and the night’s big prize, the battleground state of Pennsylvania.

But her clean sweep was denied by Sanders, who claimed a victory in Rhode Island, according to projections. He was also leading narrowly in Connecticut, according to early vote counts, but that race was still too close to call.

"Thank you Pennsylvania! What a great night," Clinton told a crowd of supporters in Philadelphia who chanted "Hillary! Hillary!"

The 68-year-old former secretary of state, seeking to become the nation’s first female commander in chief, signified her eagerness to shift toward the general election and a showdown with Republicans.

"Let’s go forward, let’s win the nomination, and in July let’s return as a unified party," she said.

Trump’s night was nothing short of huge, as he cleared 50 per cent support in all five states, according to partial vote tallies. In Rhode Island, he earned 65 per cent, with 84 per cent of precincts reporting, while Kasich earned 24 per cent and Cruz had just 10 per cent.

Most importantly, the bombastic 69-year-old New York real estate mogul extended his lead in the all-important race for the delegates who will officially choose the Republican nominee at the party’s convention in July.

"For weeks the stop Trump, dump Trump movement has tried to puncture" his rise, said James Morone, a political science professor at Brown University.

"Today’s results overwhelmingly tell you it’s not working."

’Very dumb!’

Trump’s early triumph comes in the heated aftermath of the revelation that Cruz and Kasich, desperate to prevent the frontrunner from securing the nomination before the convention, were teaming up to block him in future primary races.

Kasich agreed to forego campaigning in Indiana, a winner-take-all state that votes on May 3, and Cruz will return the favour later in New Mexico and Oregon.

But within hours of the surprise deal, Kasich -- the governor of Ohio -- was already playing it down, saying he was not telling his supporters in Indiana not to vote for him.

"This joke of a deal is falling apart, not being honoured and almost dead," Trump said on Twitter. "Very dumb!"

Clinton’s strong showing heaps pressure on Sanders, who has vowed to fight on until the California primary on June 7 and has said he refuses to accept that he has no path forward.

"Almost every national poll and every state poll has us defeating Trump and that margin for us is significantly larger than Madam Secretary Clinton," Sanders told supporters yesterday in West Virginia, which votes on May 10.

Sanders has deflected recent questions about whether he would actively support a Clinton candidacy if she is the nominee, suggesting it was up to her to win over his passionate young followers.

"This race takes a turn tonight," Trump’s national political director Rick Wiley told the network. "The common wisdom is we’re going to get to 1,237" -- the number of delegates necessary to clinch the nomination outright at the convention in July.

Trump himself had been in full attack mode a day earlier, pouring scorn on the Cruz-Kasich deal and saying "it shows how pathetic they are."

Cruz for his part sounded eager to move on to what he called "more favourable terrain."

"Tonight this campaign moves back to Indiana and Nebraska and North Dakota and Montana and Washington and California," Cruz told supporters in Indiana.

But according to a recent CBS poll, Trump leads Indiana with 40 per cent of likely Republican voters, compared to 35 per cent for Cruz and 20 per cent for Kasich.

Losing Indiana would make it much harder for Trump to gain the magic number of delegates ahead of the convention in Cleveland on July 18-21.

"If Trump takes Indiana, it’s very, very difficult to see how they can stop him," Morone said.

If he falls short of outright victory, Trump runs the risk that his delegates, most of whom are bound to vote for him in only the first round, will desert him in subsequent rounds.

Cruz and Kasich have openly said they are counting on a contested convention, where they have a shot at wooing enough delegates to snatch the nomination. — AFP